1. Field of the Invention
The present invention relates to a chemical analysis instrument having an aspirating needle and, in particular, to an analysis instrument having a movable cleaning assembly for cleaning the aspirating needle.
2. Description of The Prior Art
Most apparatus operative to perform automated chemical analysis of liquid samples use an aspirating probe, or needle, to transfer predetermined volumes of liquid sample or liquid reagent between receptacles disposed at various locations on the instrument. The aspirating probe is typically an elongated, needle-like member having a hollow passage whereby liquid may be drawn into and dispensed from the probe.
A common problem in such aspirating needle arrangements is the risk of liquid "carryover". Carryover is usually manifested as the contamination of a given reagent supply or a given sample volume by the introduction thereinto of other reagents or samples. Carryover occurs when a needle having residual traces of a previously dispensed sample or reagent is introduced into volume of a different reagent or sample.
Carryover has an additional deleterious aspect in that it may result in the introduction of an inaccurate amount of sample or reagent. The presence of additional unwanted droplets of sample or reagent on the exterior of the needle defines the usual mechanism whereby additional liquid is introduced into the destination receptacle. For especially volume sensitive analyses this aspect of carryover is particularly troublesome.
In order to eliminate or minimize the effects of carryover it has been the practice to provide wash stations at convenient locations on the instrument. The wash station may include a well-like reservoir of a purging liquid, typically water, into which the needle is immersed. Alternatively or additionally, the purging liquid may be expelled through the needle while it is located at the wash station. The wash station may optionally be provided with an arrangement whereby a jet of drying air is forced under pressure through the needle or at the exterior surface thereof. In this manner the presence of residual material on the surface or the interior of the needle is removed. The needle is inserted into the wash station at any appropriate opportunity during the operation of the probe assembly.
Typical of analysis instruments having a wash station of the above form are those described in U.S. Pat. No. 3,964,526 (Sindermann), U.S. Pat. No. 4,318,885 (Suzuki et al.) and U.S. Pat. No. 3,552,212 (Ohlin). However, in these typical situations the wash station is fixed relative to the needle. Thus, the needle must be diverted to the wash station at each occasion when cleaning is required. Although the presence of a wash station is beneficial from the carryover aspect the requirement that the needle be physically relocated to the wash station detracts from the overall throughput of the instrument.
U.S. Pat. No. 4,323,537 (Mody) discloses an analysis system having a cleaning assembly in the form of a hollow collar movable with the needle. In this system, as the needle is raised and lowered it passes through the collar. However, full immersion of the needle into a well-like reservoir is precluded due to the structure of the collar. Such a collar structure, while sufficient to contain a low velocity jet of purging liquid expelled from the probe, is unable to contain a relatively high velocity liquid jet produced when the liquid pump is cycled at high rates. Therefore, the liquid pump must be slowed to limit the velocity of the purging liquid jet expelled from the probe. As a consequence the throughput of the instrument is slowed. Furthermore, a high velocity jet of purging liquid imparts a scrubbing action to the interior of the probe. This scrubbing action is lost when the velocity of the expelled liquid jet is slowed.
Accordingly, in view of the foregoing it is believed to be advantageous to provide a cleaning arrangement which utilizes an enclosed well-like reservoir operative to purge extraneous material from the interior and the exterior of the needle while at the same time not detract from the throughput of the instrument. It is also believed to be advantageous to provide a cleaning arrangement for a probe assembly which is mechanically able to contain a high velocity purging liquid jet, thus permitting the liquid pump to cycle at its maximum rate thereby allowing the maximum scrubbing action on the interior of the probe.